Multiple patterning is a technique used in photolithographic processes to allow for greater feature density. When fabricating integrated circuits, various features such as metal lines are formed into a semiconductor substrate. To form these features, photo-masks are used to form a pattern into a photo-resist layer. For example, the regions where the photo-resist layer is removed expose the underlying substrate to an etching process used to form trenches where metal is subsequently placed.
As the patterns formed into photo-resist layers become increasingly dense, it becomes difficult to use a single photo-mask to form a pattern in the photo-resist layer because features within the nanometer range are relatively small, as compared to the resolution of a light source to which the photo-resist layer is exposed. Thus, multiple masks may be used to form the features within a pattern. Specifically, each of the multiple masks is used to create different features within the target pattern.
In some cases, a cut pattern is used in association with a main pattern to form a target pattern. The cut pattern removes features formed by the main pattern in order to achieve the desired target pattern. Using such techniques provides certain advantages to the photolithographic process. For example, it is desirable to have a large process window. The process window refers to a range of focus and exposure settings that will still produce the desired features into the photo-resist layer. A process window can be improved by keeping features within a pattern relatively uniform in density. This may involve placement of “dummy” features in or near the pattern. Dummy features are extra features that are placed in order to maintain feature density, but may not serve any function within the circuit for which the pattern is designed. To separate dummy features from real features and to create the desired pattern, a cut mask is used.
Placement of the cut features within the cut feature mask is an important consideration. If two cut features are too close to each other, then it can be difficult to form the cut features properly. Moreover, the cut features may adversely affect adjacent features. Thus, it is beneficial to consider the placement of cut features when designing a layout for a target pattern.